Machine vision vehicle alignment systems using movable cameras and targets attached to vehicle wheels, also known as “image aligners,” are well known. The targets are viewed by the cameras such that image data obtained for a prescribed alignment process can be used to calculate vehicle alignment angles for display through a user interface, usually a computer monitor. Early system implementations included rigid beams that connected the cameras so that their position and orientation with respect to each other could be determined and be relied upon as unchanging. Later system implementations were introduced comprising the use of cameras not rigidly connected to each other, but using a separate camera/target system to continuously calibrate the position of one vehicle mounted target viewing camera to another. This type of system is described in U.S. Pat. Nos. 5,535,522; 6,931,340; 6,959,253; and 6,968,282, all of which are hereby incorporated by reference herein in their entirety. An example of a vehicle wheel aligner using such image processing is the Visualiner 3D or “V3D”, commercially available from John Bean Company, Conway, Ark., a division of Snap-on Incorporated.
In order to be able to accurately measure wheel alignment angles for a vehicle using an image aligner, the wheel axis of rotation around which the target rotates must be measured, and the coordinates of the virtual wheel spindle point through which the vector passes must be determined.
The conventional method for calibrating the combined system of a target and clamp involves lifting the vehicle off the supporting surface (e.g., the shop floor or an alignment rack) so the wheels with targets mounted are able to rotate freely. The wheels are then rotated to predetermined positions to enable determination of the vector defining the wheel axis of rotation. Since the target origin traverses a circular arc, the coordinates of the center of the circle are computed from points on the circumference of the arc. The center of the circle is on the wheel axis of rotation and is referred to as the virtual wheel spindle point. The virtual wheel spindle point is projected along the wheel spindle axis to the plane of the wheel rim. The projected point is the wheel spindle point. This is illustrated in FIG. 1, wherein a vehicle 100 on whose wheel a clamp 110 carrying a target 120 is mounted. Coordinates are in a target coordinate system. The axis of rotation vector 130 passes through the wheel spindle point 140 and virtual wheel spindle point 150. The target centroid is offset from the virtual wheel spindle point 150.
Typical wheel clamp and target assemblies are manufactured so that calibration processes do not have to be repeated every time a clamp is removed from a wheel. Conventional target assemblies commonly employ self-centering wheel clamps for this purpose. Clamp mounting errors are compensated for by a well-known rolling runout calculation.
A calibration procedure for the system of target and self-centering clamp is typically performed by a technician when an aligner is first set up, using custom calibration equipment. It must be performed thereafter whenever a new target is introduced to the system; for example, when a target is replaced. Disadvantageously, the end user must wait for a service technician or, if they are to perform the calibration procedure themselves, they must have special training. Moreover, in the normal course of use, targets and their associated clamps tend to change their relative geometry (e.g., if a clamp is dropped). While the clamps and targets may still be usable, this change in relative geometry is not reflected in the original system calibration, disadvantageously resulting in a degradation of alignment accuracy over time.
A need exists for a methodology and apparatus to determine the wheel axis of rotation and wheel spindle point that does not require additional time to be taken to lift the vehicle and perform an extra process otherwise unnecessary for a typical alignment. A need also exists for a methodology and apparatus that can adjust for normal wear and tear of wheel target assemblies, to maintain alignment accuracy. Further, a need exists to minimize the cost of wheel clamps by removing the need for self-centering capability.